Conventionally, gallium nitride (GaN)-based semiconductor materials represented by a compositional formula, such as AlXGaYInZN (0≦X, Y, Z≦1; X+Y+Z=1), and having a direct-transition-type bandgap of energy corresponding to a wavelength region of short-wavelength visible light to the UV region have been employed for fabricating pn-junction light-emitting devices, such as blue, green or UV LEDs and LDs (see, for example, JP-B SHO 55-3834).
Conventionally, p-conduction-type GaN for fabricating a pn-junction gallium nitride semiconductor light-emitting device is formed so that GaN contains an additive. For example, there has been disclosed a technique in which a Group II impurity, such as magnesium (Mg) or zinc (Zn), is added to a GaN layer through ion injection means (see, for example, JP-A SHO 54-71590).
However, without any further treatment, the gallium nitride-based semiconductor layer to which a Group II impurity has been added generally does not serve as a p-type conductive layer exhibiting high conductivity. One conceivable reason for this is that hydrogen (H) migrating from a growth atmosphere to the layer during vapor phase growth electrically compensates the Group II impurity, thereby deactivating the impurity. Thus, according to a conventional procedure, a gallium nitride-based semiconductor layer is formed through addition of a Group II impurity to the layer, followed by heating the layer in order to remove, as much as possible, hydrogen contained in the layer (see, for example, JP-A HEI 6-237012). Another known technical approach is irradiation with charged particles for electrically activating a Group II impurity (see, for example, JP-A SHO 53-20882).
Even when virtually the entire amount of hydrogen is removed from the GaN-based semiconductor layer to which a p-type Group II impurity has been added, the thus obtained low-resistance p-type conductor layer does not necessarily attain excellent, reliable rectifying characteristics and electrostatic blocking voltage characteristics when a pn-junction LED is fabricated therefrom. Among these characteristics, currently, consistent electrostatic blocking voltage is difficult to attain, even when the p-type GaN-based semiconductor layer is formed on a conductive substrate, such as a silicon (Si) single-crystal substrate, silicon carbide (SiC) or gallium arsenide (GaAs).
One conceivable means for preventing local breakdown is inserting, into a semiconductor structure, a layer exhibiting wide bandgap and high resistance. However, this means has a drawback. That is, even when such a high-resistance layer is employed as, for example, a contact layer for forming an ohmic electrode and an ohmic electrode is provided on the contact layer to thereby fabricate a GaN-based semiconductor LED or LD, forward voltage (Vf) or threshold voltage (Vth) undesirably increases.
An object of the present invention is to prevent variation of electrostatic blocking voltage and increase in, for example, forward voltage of a gallium nitride semiconductor device, such as an LED, fabricated from the aforementioned conventional p-type GaN-based semiconductor layer. Particularly, the present invention provides a GaN-based semiconductor device exhibiting improved electrostatic blocking voltage, low forward voltage, etc., through causing to remain hydrogen contained in a Group-II-impurity-added GaN-based semiconductor layer in a specific region in the layer, rather than intentionally removing hydrogen to the outside of the layer, and through provision of a low-resistance region (low-resistance layer) in a layer (top portion) located on the specific region.